1. Technical Field
This invention relates in general to improvements in the field of computer systems having backup/restore or archive/retrieve subsystems. More particularly, this invention relates to a method and system for reducing the storage requirements of backup subsystems in client-server environments.
2. Description of the Background Art
In a data processing system, a backup/restore subsystem, usually referred to as backup subsystem, is typically used as a means to save a recent copy or version of a file, plus some number of earlier versions of the same file, on some form of backup storage devices such as magnetic disk drives, tapes, or optical storage devices. The backup subsystem is used as a means of protecting against loss of data in a given data processing system. For example, if an on-line version of a file is destroyed or corrupted because of power failure, hardware, or software error, user error or some other type of problem, the latest version of that file which is stored in a backup subsystem can be restored and therefore the risk of loss of data is minimized. Another important use of backup subsystems is that even if failures do not occur, but files or data are deleted or changed (either accidentally or intentionally), those files or data could be restored to their earlier state thus minimizing the loss of data.
Therefore, it can readily be apparent that backup subsystems are and will remain an important part of the field of data processing.
A closely related concept to the backup subsystem is a method and system called archive/retrieve, usually referred to as an archive subsystem. Archiving refers to making copies of files on lower cost storage such as tape so that files can be deleted from more expensive technology such as disk storage. Since disk storage is frequently being updated, an archival copy also allows the state of a collection of data to be captured for later reference, even if the primary copy of the data is not going to be deleted. An example would be the archiving of a set of financial data at the end of a fiscal period. Although the improved method of carrying out the backup disclosed in this application is primarily described for a backup system, it will be obvious to the person of ordinary skill in the art of data processing that the systems and methods described herein are also applicable to archive systems or other related storage management systems.
At the present time, the majority of backup systems run on host systems located in a data processing environment. Typically, a new version (also referred to as changed version) of a file is backed-up based on a predetermined schedule such as, at the end of each day, or after each time that a file has been updated and saved.
Backup systems generally consume large amount of storage media because multiple versions of large amounts of data are being backed up on a regular basis. Therefore, those engaged in the field of data processing and especially in the field of backup/restore systems are continuously striving to find improved methods and systems to reduce the storage demand in backup systems. Current backup systems typically utilize one or both of the following methods to enable the storage of and retrieval of multiple versions of a given file. These are: (1) the full backup method and (2) the incremental backup method.
The full backup method is the most basic method used which requires the backup of an entire collection of files, or a file system, regardless of whether individual files in that collection have been updated or not. Furthermore, in the full backup method multiple full versions of each file are maintained on a storage device. Since maintaining multiple full copies of many files consumes substantial amount of storage, some type of compression technique is sometimes used to reduce the amount of data stored. Compression techniques basically rely on the presence of redundancy within the file, so called intra-file redundancy, in order to achieve this reduction. The most common method is the use of a method of file compression known as Lempel-Ziv method (also known as Adaptive Dictionary Encoder or LZ coding) described in a book by T. C. Bell et. al., titled Text Compression, pp 206-235. The essence of Lempel-Ziv coding is that phrases are replaced with a pointer to where they have occurred earlier in the text, thereby saving the storage space associated with multiple occurrence of any given phrase. This is a general method which can be applied to any file and typically results in compression ratios of the order of between 2 and 3.
Incremental backup method is an alternative to full backup used in backup systems where only those files, in any given collection of files, are backed up which have been changed since the previous incremental or full backup.
It is apparent to those skilled in the art that in any given backup system, the higher the backup frequency, the more accurately the backup copy will represent the present state of data within a file. Considering the large volume of data maintained and continuously generated in a typical data processing system, the amount of storage, time, and other resources associated with backing up data are very substantial. Thus, those skilled in the art are continuously engaged in searching for better alternatives and more storage and time efficient systems and methods for backing up data.
Aside from the compression technique which is heavily utilized to reduce storage requirement in a backup system, there exists a quite different method of achieving reduction in file size, known as delta versioning. Delta versioning has never been used in any backup system.
Delta versioning which is also referred to as "differencing" or "deltaing" relies on comparison between two files where multiple version of a file is saved in a form of a "base" file, also called a "base version" of a file, together with predetermined number of small files which represent only the changes to the base file. The small files, also referred to as "delta" files or "difference" files, contain the difference or delta from the base file. Delta files are generated as a result of comparing the base file with a later (newly arrived) or an earlier version of the base file. Thus this method of storage reduction exploits redundancy between files, or "inter-file" redundancy, in order to achieve reduction in storage requirement. This method which is used in the software art of Source Code Control Systems, discussed in a reference below, can provide substantial storage saving in backup systems, since frequently the selection of a file for incremental backup occurs after a small change has been made to that file. Therefore, since many copies are frequently made in backup systems to files that differ only slightly from one another, the differencing method offers great potential for substantial reduction in the amount of data stored in backup subsystems.
At the present time none of the backup systems that use compression techniques utilize delta versioning. Moreover, no one has ever invented a method and system allowing the use of compression and delta versioning together in the same backup system.
Delta versioning falls into two general classes: one is where the base file is the oldest version of a file and the delta files represent newer versions. This method is referred to as "forward" deltas. The other is where the base file is the latest version of a file and the delta files represent older versions. This method is referred to as "reverse" deltas. The "reverse" delta is the more common method because usually the most utilized version of a file is the last version created.
A technical paper by M. J. Rochkind, titled "The Source Code Control System", IEEE Transaction on Software Engineering, Vol. SE-1, No. 4, December 1975, PP 364-370, teaches a software tool, known as source code control system (SCCS) which is designed to help managing changes to a source code (source program) in the field of software development tools. In SCCS environment, every time a module (file) is changed the change is stored as a discrete delta where the space required to store a delta is only slightly greater than the amount of text inserted by that delta. However, Rochkind does not teach or suggest the use of delta files in a backup and archiving subsystem in either a central or a client-server environment as a means for reducing the storage requirements of such subsystems.
U.S. Pat. No. 4,912,637 issued on Mar. 27, 1990 to C. R. Sheedy et al., teaches a system for preserving, generating, and merging various versions of the same file by a modified delta method. Sheedy teaches using an indexed line file where every line active in any version of a given file is stored, together with a variant history file where the history of the status of each line in various versions is recorded. Using these two files, any desired version of a program may be generated directly without the need for creating any of the intermediate versions. However, Sheedy does not teach or suggest the use of this modified method in backup and archiving systems in either a central processing or a client/server environment as a means for reducing the storage requirement of a backup system.
U.S. Pat. No. 5,263,154, issued on Nov. 16, 1993 to L. E. Eastridge et al., teaches a method and system for incremental backup copying of a file in a data processing system which minimizes the suspension of the data processing system during such backup copying. This is done by first physically backing up a data set on a storage subsystem on a scheduled or opportunistic basis. Thereafter, creating side-files of the data set modified. The side-files are then used in the next scheduled or opportunity to update the backed-up data set. However, Eastridge does not teach or suggest the use of delta files as a means of minimizing storage requirement in a backup and archiving subsystem in either a central processing environment or a client-server environment.
U.S. Pat. No. 5,278,979 issued on Jan. 11, 1994 to R. D. Foster, et al., teaches a method and system in the field of software code development known as Single Entity Versioning where by creating and maintaining a unique version identification and a control data file, multiple versions of source data is efficiently stored in a single entity. However, Foster does not teach or suggest the use of delta versioning or Single Entity Versioning as a means of minimizing storage requirement in a backup and archiving subsystem in either a central processing environment or a client-server environment.
Considering that the amount of the data generated on the daily basis by the computers is growing at a very fast rate, there is a need for an improved and innovative method and system to reduce the storage requirements of backup systems in central data processing systems and further in client-server environments which as will be discussed below present unique backup issues.